Device for reducing the output current of a battery charger

ABSTRACT

A device for reducing the output current of a primary switched battery charger, which charger includes an input DC power circuit, a high frequency transformer and control unit for modulating the DC input power. The device includes elements for measuring pulse ratio of switch pulses on the output side of the charger, elements for measuring peak value of output voltage, elements for differentially amplifying the signals measured and elements for integrating voltage/current of the differentially amplified signals, wherein the integrated voltage/current is used for modulating the input DC power in order to reduce the output current. The device includes elements for measuring the output effect of the charger, elements for measuring the output voltage of the charger, elements for dividing the signals measured and elements for multiplexing voltage/current of the output from the elements for dividing, wherein the multiplexed voltage/current is used for modulating the input DC power in order to reduce the output current.

TECHNICAL AREA

The present invention relates to a device for controlling the current ofa battery charger, and in particular a primary switched charger.

BACKGROUND OF THE INVENTION

The charging of batteries should be performed as carefully as possiblein order not to damage the battery or reduce its capacity and at thesame time the charging should be performed as quickly as possible, thesetwo criteria cannot always be obtained at the same time. In order toshorten the charging time, the charging current can be increased. A highcharging current can however damage the battery. To this end the batterymanufacturers have strict instructions on how high a charging current isallowed to be as a function of battery size. A normal recommendation isthat the charging current is obtained by multiplying the capacity of thebattery in ampere hours with 0, 1, ie a 20 Ah battery should be chargedwith 2 A.

A battery charger is therefore “locked” in a window between being toosmall, with long charging time, and too large, with detrimentalinfluence on the battery life. As a consequence of this, users with anumber of different battery sizes are forced to have a number ofdifferent battery chargers, alternatively they charge outside therecommendations of the manufacturers. For all charging of batteries, thefinal charging voltage has a large impact on the life of the battery. Atoo high voltage means that the battery develops gas with an increasedconcentration of sulphuric acid and accelerated grid corrosion as aconsequence. A too low voltage means an uncompleted charging withpartial sulphating and lost battery capacity as a consequence. A thirdparameter is that current ripple shall be kept low as it cause anincrease in the battery temperature during charging with a decreasedlife as a consequence.

On linear chargers, ie chargers arranged with a transformer thatconverts the mains voltage to charging voltage, which are thepredominant type for chargers, the current may be controlled in that thetransformer is provided with several primary windings and the outputvoltage is varied by choice of primary winding. Due to this, the outputvoltage is altered and because the current is proportional to thevoltage, the current can be affected. The drawback with this type isapparent on an unregulated linear charger because the voltage to thebattery can rise to levels which are detrimental to the battery if quickcharging is required and a too low voltage level if it is desired toreduce the current. Ripple is something undesirable on a charger due tothe above mentioned problems. The development trend regarding batterychargers is a transfer to primary switched devices, which offer a moreexact control of voltage and current and at the same time smallerdimensions. There exists today, as far as the inventors are aware, nochangeable output currents on primary switched chargers.

BRIEF DESCRIPTION OF THE INVENTION

The aim of the present invention is to provide a primary switchedcharger where the user can choose one or more current levels. Because ofthis, charging parameters can be adapted in order to provide the righttreatment of the battery and one single charger can be adequately usedfor different types and sizes of batteries.

According to one aspect of the invention it discloses a device forreducing the output current of a primary switched battery charger, whichcharger comprises an input DC power circuit, a high frequencytransformer and a control unit for modulating the DC input power,characterised in that it comprises means for measuring pulse ratio ofswitch pulses on the output side of the charger, means for measuringpeak value of output voltage, means for differentially amplifying thesignals measured and means for integrating voltage/current of thedifferentially amplified signals, wherein the integrated voltage/currentis used for modulating the input DC power in order to reduce the outputcurrent.

According to second aspect of the invention, a device for reducing theoutput current of a primary switched battery charger is disclosed. Thedevice for reducing the output current of a primary switched batterycharger according the second aspect of the present invention comprisesan input DC power circuit, a high frequency transformer and a controlunit for modulating the DC input power, characterised in that itcomprises means for measuring the output effect of the charger; meansfor measuring the output voltage of the charger; means for dividing thesignals measured; and means for multiplexing voltage/current of theoutput from the means for dividing, wherein the multiplexedvoltage/current is used for modulating the input DC power in order toreduce the output current.

According to a third aspect of the invention, there is provided a methodfor reducing the output current of a primary switched battery charger,which charger comprises an input DC power circuit, a high frequencytransformer and control unit for modulating the DC input power. Themethod is characterised in that it comprises the steps of: measuring theoutput effect of the charger; measuring the output voltage of thecharger; dividing the signals measured; and multiplexing voltage/currentof the output from the means for dividing, wherein the multiplexedvoltage/current is used for modulating the input DC power in order toreduce the output current.

According to a fourth aspect of the invention, a method for reducing theoutput current of a primary switched battery charger, which chargercomprises an input DC power circuit, a high frequency transformer and acontrol unit for modulating the DC input power is disclosed. The methodis characterised by the steps of: measuring pulse ratio of switch pulseson the output side of the charger; measuring peak value of outputvoltage; differentially amplifying the signals measured; integratingvoltage/current of the differentially amplified signals, wherein theintegrated voltage/current is used for modulating the input DC power inorder to reduce the output current.

According to a further aspect of the invention there is providedComputer readable medium comprising instructions for bringing aprogrammable device to perform the method according to the third aspectof the invention or the fourth aspect of the invention.

The advantage with the present invention is that the design provides agood possibility to reduce the charging current to the battery on theoutput side and is at the same time very cost efficient. A currentlimitation is possible to perform in the interval of around 5% to 30% ofthe maximum current. The current regulation will partly be reciprocallyproportional to increased load, ie the charging current decreases withincreased load. Maximum current is therefore obtained just before thevoltage regulation cuts in during reduced load.

These and other aspects of, and advantages with, the present inventionwill become apparent from the detailed description of the invention andfrom the accompanying drawings.

SHORT DESCRIPTION OF THE DRAWINGS

In the following description of the invention, reference will be made tothe following drawings, of which

FIG. 1 shows a circuit configuration of a battery charger comprising thecurrent device according to the present invention,

FIG. 2 shows a block diagram of the current device of the presentinvention,

FIG. 3 shows a more detailed illustration of the effect regulationcircuitry shown in FIG. 2, and

FIG. 4 shows a circuit configuration of the control unit of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The battery charger shown in FIG. 1 is a primary switched chargercomprising in a known manner a DC power circuit 8 connectable to themains, a diode bridge 10, a smoothing capacitor 12 and a high frequencytransformer 14 having a primary winding 14 a connected to the DC powercircuit 8 and a secondary winding 14 b. The smoothing capacitor storesenergy as a high DC voltage. The transformer transforms the high voltageto a charging voltage. A control unit 16 comprising, inter alia, anelectronic switch, like a field effect transistor FET, is arranged tothe DC power circuit and the transformer capable of chopping up the DCpower from the DC power circuit into pulses, and controlling andmodulating the signal. Furthermore, the control unit 16 comprisesmodulation circuitry arranged for the modulation of the signal. In FIG.4 a circuit configuration of the control unit 16 will be shown.

On the output side of the high frequency transformer 14 are two lines,positive 18 and negative 20, provided with means to connect to a battery21. A rectifying element 22, such as a diode, is arranged to thepositive line, and a smoothing capacitor 24 is arranged between thepositive and negative line.

The device according to the invention for providing a low currentcomprises a circuit 26 for measuring the pulse ratio of switch pulses onthe output side of the high frequency transformer 14, connected via aline 28 to the positive line 18. The pulse ratio is the ratio betweenthe pulse duration and the interval between two consecutive pulses andis a measure of the output effect of the battery charger at a givenoutput voltage. Accordingly, the circuit 26 in fact measures the outputeffect of the battery charger. It further comprises a circuit 28 formeasuring peak values of the output voltage, connected between thepositive and the negative lines. Since the output voltage is a DCvoltage which may comprise superimposed voltage components, the circuit28 for measuring the peak values preferably is arranged to measure theeffective values of the output voltage. Output signals from the pulseratio circuit 26 and the peak value circuit 28 are fed via lines 30, 32to a differential amplifier 34 for the peak values/pulse ratios arrangedto compare the peak value voltage and the pulse ratio voltage anddifferentially amplifying the resulting value. Preferably, the amplifier34 is arranged to reverse output signal.

An integrating amplifier 36 for feed-back of voltage/current is arrangedwith an input line 38 from the positive line 18. A second input line 40provided with a breaker 42 is provided from the amplifier 34 foractivating/deactivating a low-current area. A feed-back line 44 isprovided from the integrating amplifier 36 to the control unit 16 to beused for PWM. Preferably, an isolating element (see FIG. 3) is arrangedat the feed-back line 44 between the integrating amplifier 36, forexample, an optical-coupler or a transformer.

FIG. 2 shows a block diagram of the components of the differentcircuitry comprised in the present invention.

Furthermore, the pulse ratio circuit 26 comprises a diode 50 connectedto line 28 and a resistor 52 in series with the diode. The resistor isin turn connected to a second resistor 54 and a capacitor 56 arranged inparallel with each other and connected to earth.

The peak value circuit 28 comprises two resistors 58 and 60 connected inseries with each other and between the positive line 18 and the negativeline 20.

The amplifier 34 comprises a transistor 62 where its emitter isconnected via line 64 between the resistors 58, 60 of the peak valuecircuit 28. The base of the transistor 62 is connected via a resistorbetween the resistor 52 and the resistor/capacitor 54, 56 of the pulseratio circuit 26. Further, the collector of the transistor 62 isconnected to a resistor 68 and a capacitor 70 arranged in parallel witheach other and connected to earth.

The integrating amplifier 36 comprises a transistor 72, where its baseis connected to the collector of the transistor 62. The emitter of thetransistor 72 is connected to earth via a resistor 74. The collector ofthe transistor 72 is connected to the base of a second transistor 76.The collector of the second transistor 76 is connected to the positiveline 18 via a line 78 and a resistor 80. A further line 82 is arrangedbetween the line 78 and the collector of the first transistor 72, whichline is arranged with a resistor 84.

Preferably, an optical-coupler 37 (see FIG. 3) is arranged at thefeed-back 44 between the integrating amplifier 36 and the control unit16 to performs the transfer of the feed-back signal from the integratingamplifier 36 at the secondary side to the primary side. Theoptical-coupler 37 comprises, inter alia, a LED (not shown).

Furthermore, the integrating amplifier 36 is connected to effectregulation circuitry, which is indicated by the dashed box 39,comprising a resistor 86 and a resistance adjusting means 88 andregulation circuitry (see FIG. 3). Of course, there are a number of waysof implementing a resistance adjusting means as the man skilled in theart realizes, for example, by means of a trimming potentiometer. Theresistor 86 is connected to the line 78 and to the emitter of the secondtransistor 76. In turn, the trimming potentiometer 88 is connectedbetween the connection of the resistor 86 and the emitter of the secondtransistor 76 and earth. The feed-back line 44 is connected to theconnection between the resistor 86 and the trimming potentiometer 88.The output voltage is determined by the voltage division between theresistor 86 and the trimming potentiometer 88. Accordingly, the outputvoltage can be adjusted by an operator by changing the resistance valueof the trimming potentiometer 88.

The breaker 42 for activating/deactivating the low-current areacomprises a transistor 90 with its collector connected between thecollector the first transistor 72 and the base of the second transistor76 of the integrating amplifier 36. The emitter of the transmitter 90 isconnected to earth. The base is via a resistor 92 connected to anelectronic switch so that the low-current area of the charging can bechosen by an operator. The skilled man realizes that there are a numberof ways to implement a breaker and that the embodiment shown is intendedonly as an example.

Turning now to FIG. 3, a more detailed illustration of the effectregulation circuitry of FIG. 2 is shown. The regulation circuitry 39comprises a regulator 100, which preferably is an integrated shuntregulator. The shunt regulator 100 is connected to the resistor 86, theamplifier 36 (or in fact, the emitter of the transistor 76), thetrimming potentiometer 88, a second resistor 102 and a third resistor104. Furthermore a capacitor 106 is connected to a connection betweenthe shunt regulator 100 and the second resistor 102 and to a connectionbetween the resistor 86 and the trimming potentiometer 88. The thirdresistor 104 is in turn connected to the optical-coupler 37 and to afourth resistor 108. The resistance values of the second, third andfourth resistors 102, 104 and 108 are carefully selected in order toadapt the feed-back qualities of the regulating circuitry.

Referring now to FIG. 4, a circuit configuration of the control unit ofFIG. 1 will be shown. The control unit 16 comprises an effect limitingcircuit 120, an electronic switch 122, like a field effect transistorFET, and a modulation circuit 124. Furthermore, the effect limitingcircuit 120 includes a first resistor 126 connected to the electronicswitch 122 and to earth and a second resistor 128 connected to the firstresistor 126, to the optical-coupler 37 and to the modulation circuit. Acapacitor 130 is arranged in parallel with the first resistor 126 andconnected to earth. The details of the modulation circuitry will not bedescribed in detail here, because it do not form part of the presentinvention and its function and design is well known to the man skilledin the art. Preferably, the signal is modulated using pulse widthmodulation (PWM). Of course, the present invention can be used with anumber of other modulation methods, for example, pulse-positionmodulation (PPM) or pulse frequency modulation (PFM). In such cases, anynecessary modifications of the circuits of the current device of thepresent invention in order to adapt the current device to the modulationmethod used are easily performed by the skilled man and are thereforenot described herein.

In function, the device according to present invention operates asfollows. The output voltage of the battery charger is measured anddivided to a level suitable for the amplifier 34 in the peak valuecircuit 28. The peak value is used to compensate the measurement of thepulse ratio. Of course, other values are possible to use as, forexample, the effective value. At an increased charging current, theoutput voltage of the charger will decrease.

In the pulse ratio circuit 26, the pulse ratio, i.e. the ratio betweenthe duration of the pulse and the period of time between two consecutivepulses, is measured and, in addition, the pulses are rectified. Thevoltage at the capacitor 56 increases as the output effect of thecharger increases.

Then, the pulse ratio voltage and the peak value voltage is compared inthe amplifier 34, the pulse ratio is subtracted from the peak value andthe resulting value is amplified. Thus, the amplifier can be seen as adividing circuit. Preferably, the output signal is inverted in theamplifier 34. This is advantageous in order to achieve a correct startup since the current will increase at a slow rate, i.e. a soft start isobtained. A decreased pulse ratio entails an increased output signal,i.e. an increased voltage at the capacitor 70.

In the integrating amplifier 36 the present output voltage is integratedwith the output signal from the amplifier 34, or, in fact, the presentoutput voltage is added to the output signal from the amplifier 34.Thus, the integrating amplifier 36 can be seen as multiplexing circuit.An increased input signal, i.e. an increased input voltage, receivedfrom the amplifier 34, at the base of the first transistor 72,corresponds to a decreased output current, which entails that theresistor 84 gradually will be disconnected. This, in turn, leads to anincreased output voltage of the charger and, thereby, an increasedoutput current, i.e. charging current.

The regulation of the output voltage of the charger is effected by meansof an active limitation of the output effect of the charger. Asmentioned above, the output voltage is determined by the ration betweenthe resistance values of the resistor 86 and the trimming potentiometer88. In effect, the voltage of the cathode of the regulator 100 will beself-regulated so that the voltage at the control input of the regulatorwill set to approximately 2.5 V. If the output voltage of the chargerincreases, the regulator 100 will draw a larger current through thecathode. Thereby, the current through the third resistor 104 isincreased. This increased current leads to an activation of the LED ofthe optical-coupler 37, which, in turn, decreases the output effect ofthe charger.

The transfer of the feed-back signal from the integrating amplifier 36of the current device, arranged at the secondary side of the charger, tothe effect limiting circuit 120 arranged in the control unit 16 at theprimary side is performed, as mentioned above, by means of anoptical-coupler 37.

In the effect limiting circuit 120, the current through the electronicswitch 122 and the high-frequency transformer 14 is measured with, forexample, a primary shunt circuit (not shown). At a given current level,which mainly is determined by the resistance of the first resistor 126,the pulse at the control input of the electronic switch 122 is disabledor interrupted. Consequently, the maximal allowed output effect of thecharger is determined by means of the resistance value of the firstresistor 126. Furthermore, the signal, i.e. the pulse, is filtered bythe second resistor 128 and the capacitor 130 in order to removeundesired disturbance peaks in the signal. By adding the voltage at thecapacitor 130 via the optical-coupler 37 and the feed-back from thevoltage/current control of the secondary side, the output effect of thebattery charger can be adjust at each point of time and, thereby, thedesired charging voltage or current can be obtained.

As mentioned above, in this preferred embodiment, the modulationcircuitry 124 utilizes pulse width modulation. The modulation circuitrycontrols the control input of the electronic switch 122 with a givenfundamental frequency of the pulse operation. The fall of the pulseoccur when the voltage has reached a certain level. The period of timebetween the operating and fall of the pulse determines the duration ofthe pulse and accordingly the magnetic energy stored in the primarywinding of the high-frequency transformer 14 at each pulse. During theduration between to consecutive pulses, the stored energy is transferredto the secondary side of the circuit via the secondary winding of thehigh-frequency transformer 14 (i.e. a Fly-Back converter). Preferably, afundamental frequency of approximately 50 kHz is used. Of course, thecurrent device according to the present invention can be used with othertypes of converters, for example, a forward-converter, as well as, withother frequencies.

The design according to the invention provides a good possibility toreduce the charging current to the battery on the output side and is atthe same time very cost efficient. A current limitation is possible toperform in the interval of around 5% to 30% of the maximum current. Thecurrent regulation will partly be reciprocally proportional to increasedload, ie the charging current decreases with increased load. Maximumcurrent is therefore obtained just before the voltage regulation cuts induring reduced load.

Although specific embodiments have been shown and described herein forpurposes of illustration and exemplification, it is understood by thoseof ordinary skill in the art that the specific embodiments shown anddescribed may be substituted for a wide variety of alternative and/orequivalent implementations without departing from the scope of thepresent invention. This application is intended to cover any adaptationsor variations of the preferred embodiments discussed herein.Consequently, the present invention is defined by the wording of theappended claims and equivalents thereof.

As an example, many of the functions described above may be obtained andcarried out by suitable software comprised in a micro-chip, an ASIC, orthe like data carrier.

1. Device for reducing the output current of a primary switched batterycharger, which charger comprises an input DC power circuit, a highfrequency transformer and a control unit for modulating the DC inputpower, said device comprising: a pulse measuring unit for measuring apulse ratio of switch pulses on the output side of the charger; a peakmeasuring unit for measuring a peak value of an output voltage of thecharger; a differential amplifying unit providing an output signal bydifferentially amplifying the measured pulse ratio of switch pulses fromthe pulse measuring unit and the measured peak value of the outputvoltage from the peak measuring unit; and an amplifier for adding theoutput voltage of the charger to the output signal from saiddifferential amplifying unit, wherein the device is adapted to transmitthe output signal from said amplifier to said control unit for use inmodulating the input DC power in order to reduce the output current ofthe charger.
 2. Device according to claim 1, characterised in a switchcapable of switching on and off a connection between said amplifyingunit and said amplifier.
 3. Device according to claim 1, wherein, thepulse measuring unit comprises connections for for connection to theoutput side of the high frequency transformer of the charger, themeasured pulse ratio is a ratio between a pulse duration and an intervalbetween two consecutive pulses and is a measure of output effect of thecharger at a given output voltage, the peak measuring unit comprisesconnections between positive and the negative lines of the charger, thedifferential amplifying unit connected to outputs of the pulse measuringunit and the peak measuring unit to receive the measured pulse ratio ofswitch pulses from the pulse measuring unit and the measured peak valueof the output voltage from the peak measuring unit, the amplifierconnected to receive the output signal from the differential amplifyingunit and the output voltage of the charger, an output of the amplifierconnected to said control unit by a feedback line, the amplifiertransmitting the output signal from the amplifier to said control unitvia said feedback line for use in modulating the input DC power in orderto reduce the output current of the charger.
 4. Method for reducing theoutput current of a primary switched battery charger, which chargercomprises an input DC power circuit, a high frequency transformer and acontrol unit for modulating the DC input power, the method comprisingthe steps of: measuring a pulse ratio of switch pulses on the outputside of the charger; measuring a peak value of an output voltage of thecharger; differentially amplifying the measured pulse ratio of switchpulses and the measured peak value of the output voltage to obtain adifferential amplification output signal; adding the output voltage ofthe charger to the obtained differential amplification output signal toobtain a control signal; and transmitting the control signal to thecontrol unit for use in modulating the input DC power in order to reducethe output current of the charger.
 5. A combination, comprising: aprimary switched battery charger, said charger comprising an input DCpower circuit, a high frequency transformer, and a control unit formodulating the DC input power; and a device for reducing an outputcurrent of the primary switched battery charger, said device comprisingi) a pulse measuring unit connected to the output side of the charger tomeasure a pulse ratio of switch pulses on the output side of thecharger; ii) a peak measuring unit connected to the output side of thecharger to measure a peak value of an output voltage of the charger;iii) a differential amplifying unit connected to output sides of thepulse measuring unit and peak measuring unit to provide an output signalby differentially amplifying the measured pulse ratio of switch pulsesfrom the pulse measuring unit and the measured peak value of the outputvoltage from the peak measuring unit; and iv) an amplifier connected tothe output signal of the differential amplifying unit and to the outputside of the charger and adding the output voltage of the charger to theoutput signal from said differential amplifying unit, the amplifierhaving an output connected to said control unit, wherein the amplifiertransmits the output signal from the amplifier to said control unit,said control unit using the output signal from said amplifier inmodulating the input DC power in order to reduce the output current ofthe charger.